This invention relates to a pumping and filtering system for use in dispensing precise amounts of filtered liquid at precise rates. In particular, the invention relates to a pump for dispensing liquids such as photoresist liquids which are used to manufacture semi-conductors.
U.S. Pat. No. 4,950,134 shows a pump which is useful in practicing the present invention. The pump described in U.S. Pat. No. 4,950,134 is a positive displacement electro-hydraulically operated diaphragm pump particularly useful in applying photoresist liquid during the manufacture of semiconductor wafers. The content of U.S. Pat. No. 4,950,134 is incorporated herein by reference.
The need for precision and careful deposition of photoresist liquids during the manufacture of semi-conductors is critical. Wafers, usually about 4 to 8 inches in diameter, are positioned beneath a dispense nozzle. An initial amount of photoresist liquid sufficient to form an initial layer of about 1.5 to 5 milliliters, according to the size of the wafer, is deposited. The wafer is then rotated in order to spread the liquid evenly over the surface of the wafer. The final thickness of the layer of photoresist liquid is measured in thousands of Angstroms.
In some systems, photoresist liquid is deposited in the center of the wafer when the wafer is motionless. In such static dispensing systems, a substantial amount of the photoresist liquid is spun from the wafer and discarded, wasting much of the photoresist liquid initially deposited on the wafer. However, a more efficient use of photoresist liquid occurs in dynamic dispensing systems in which the photoresist is deposited on a rotating wafer. The need for precision in the duration, amount and rate of deposition is even more critical in dynamic dispensing as opposed to static dispensing. Motion of the wafer in a dynamic dispense system causes the photoresist liquid to be dispensed in a predetermined pattern. Therefore, dispensing generally, and the dispense rate in particular, must be tightly controlled so that the dispense patter is consistent from one wafer to the next.
The liquids which are dispensed with pumps such as the one shown in U.S. Pat. No. 4,950,134 are preferably filtered through filters having very small openings, i.e. on the order of 0.1 to 0.2 microns. As a result, pressure of between about 3 and 7 psi are required to drive the liquid through such filters. However, in precisely dispensing liquids, it has been found advantageous to draw backward or "suckback" liquid at the dispensing nozzle. This is because as flow is stopped, drips, or potential drips, may form at the dispense nozzle. If a drop of photoresist liquid falls on a semi-conductor wafer after the desired amount has been dispensed, the coating on the wafer can become deformed, in which case the wafer may be reprocessed. This problem is present when the wafer remains directly beneath the dispense nozzle as the photoresist is dispersed by rotation of the wafer and during movement of the wafer away from the area beneath the nozzle.
The use of a separate line for purposes of conveying suckback liquid allows pressure to be maintained in the main dispensing line. It is particularly important to maintain pressure on the liquid in the area of the filter, because air bubbles, which are unavoidably present in the filter medium, will decompress if pressure is removed. This decompression and the subsequent recompression would, if allowed to occur, distort the measurement of the amount of liquid to be dispensed.
It is therefore an object of the present invention to provide a pumping system which is capable of dispensing precise amounts of liquid at predetermined rates.
Another object of the present invention is to provide a pumping system that repeatably disperses precise amounts of liquid at precise rates for precise durations.
A further object of the present invention is to provide a pumping system which facilitates the suckback of liquid through the system.
Still another object of the present invention is to provide a pumping system in which the useful life of the filters used in the system is improved.
Yet another object of the present invention is to provide a pumping and filtering system for liquids in which the liquid flows in only one direction through the filter.
Still another object of the present invention is to provide a pumping and filtering system in which variations in pressure on the liquid in the area of a filter is minimized.
A further object of the present invention is to provide a dispensing system in which the amount and rate of dispensing is repeatable within small tolerances over many cycles.
These and other objects of the present invention are achieved with a pumping system in which an electro-hydraulic diaphragm pump is used to withdraw liquid from a source bottle, and, in turn, drive the source liquid through a filter medium to a dispensing port. After a particular amount of the source liquid has been directed through the filter, the pump is used to draw the liquid away from the dispensing nozzle through a suckback line around the filter. This arrangement allows for the maintainence of generally constant pressure on the liquid, thus avoiding decompression and expansion of gases trapped in the filter medium.
A better understanding of the present invention will be obtained upon a reading of the following specification read in conjunction with the accompanying drawings, wherein: